The present invention relates to a method of dispensing liquid solder in separate portions; an apparatus for performing the method; and an interchangeable moulding die for use with the apparatus.
Soldering methods of this kind are typically (but not exclusively) used for bonding of semiconductor elements (chips) on a metal support (lead frame). In particular, power semiconductors are as a rule attached by soft solder to the substrate, usually of copper. The soldered joint ensures that heat produced in the semiconductor chip when in operation is dissipated more efficiently than it would if the joint were made of adhesive. However, particularly in case of increased power density, the soldered joint must be perfectly homogeneous, i.e. it should be of the specified thickness and evenly distributed, and the layer of solder should wet the whole area of the chip evenly, be entirely free from air cavities and from impurities. At the same time, as far as possible, the solder should not protrude from the edges nor spread beyond the area of the chip, and this in turn requires that an exact amount of solder is dispensed and is accurately positioned on the substrate. mass of liquid solder, the suitability of this apparatus for continuous operation seems doubtful.
More generally in the field of soft soldering, arrangements are known with conventional hand-held soldering irons and other similar soldering equipment, where a solder wire is progressively fed in a guidance system through a longitudinal bore to a perforated soldering tip preheated to the solder's melting temperature, and molten solder is forced out through a narrowed outlet in the soldering tip or is allowed to flow out by capillary action (e.g. DE-A-36 12 341, DE-C-37 34 550, AT 291 716, U.S. Pat. No. 2,875,719). In these the solder is melted in the soldering tip itself, whereby the amount of solder is indeterminate. In heating the soldering tip, the heat flow is generally from the rear towards the tip, hence there are usually heat-insulating guide sleeves fitted immediately behind the soldering tip or the heat source to inhibit heat flow toward the rear.